1. Field of the Invention
This invention relates to printed circuit boards and more particularly to the placement and connection of a connector with respect to line traces.
2. Description of the Related Art
Packaged integrated circuits (ICs) are commonly interconnected on printed circuit boards (PC boards or PCBs) by layers of conductors (commonly referred to as line traces). Each line trace carries signaling between ICs. A line trace can interconnect multiple devices or make a point to point connection between two ICs. Modern PC boards can have as many as 20-24 layers of line traces with pitch spacings as small as 4 MILs. However, in the personal computer industry, cost and other restrictions keep the number of layers on the main PC board at four. As technology progresses, the signal transfer rate between ICs over these line traces increases. Additionally, the signaling voltage decreases with improvements in technology. High-speed, low-voltage signaling requires strict adherence to routing rules, for example, maximum routing lengths of line traces and maximum trace widths, to ensure signal quality.
To test or debug a PC board system including integrated circuits, the signaling on line traces of the PC board is observed. Often, stub traces are formed off of the line traces and routed to a connector, typically near the edge of the board, to provide visibility. A logic analysis tool can be attached to the traces via the connector, providing a testing platform. A stub trace can change the electrical characteristics of the line trace, decreasing the signal quality. For example, the stub trace can cause effects such as reflections or dispersions. For typical applications that have a bus speed less than 50 MHz, stub traces routed to a connector have not been a problem. For complex applications that have high-speed and/or low-voltage signaling, the addition of a stub trace can require the system to be tested at slower than operational speeds. For example, the HyperTransport bus which has bus speeds from 200 to 800 MHz cannot have stub lengths greater than ½ inch. Anything greater will cause errors in the signaling. Thus, testing results for complex applications are less reliable.
Because connectors are expensive and trace stubs affect signal integrity, separate boards for test and production are typically manufactured. Often, the production board has a different layout than the test board. Because two types of boards are made, the system cost is increased. Additionally, because there might be some electrical inconsistencies between the test and production boards, the reliability of the testing function is decreased.
A new technique for observing signaling on the traces between ICs on a PC board is needed that enables signals to be transmitted over line traces while encountering limited signal degradation. The present invention addresses these and other problems by providing a technique for mechanical interconnection into the line traces on a PC board without causing significant signal degradation and without increasing system cost.